1. Field of the Invention
The present invention relates to an optical system. More specifically, the present invention relates to a photographic optical system of an image pickup apparatus.
2. Description of the Related Art
It is desired by the market that a photographic optical system of a single-lens reflex digital camera or a single-lens reflex film camera has a wide angle of view and a long back focal length. U.S. Pat. No. 5,559,638 discusses a retrofocus photographic optical system as a photographic optical system having a wide angle of view and a long back focal length.
In such a retrofocus photographic optical system, a lens unit having an overall negative refractive power is disposed on the front portion of the photographic optical system (on the object side in a photographic optical system of a camera). Furthermore, a lens unit having an overall positive refractive power is disposed on the rear portion of the photographic optical system (on the image side in a photographic optical system of a camera).
An optical system having a wide angle of view and a long back focal length can be implemented by a configuration described above, in which the overall refractive power is asymmetrically arranged. The degree of asymmetry of the refractive power may become higher as the angle of view becomes wider. If the degree of asymmetry of the refractive power becomes high, various aberrations, such as coma, astigmatism, and spherical aberration, are likely to occur in a large amount.
For example, in a retrofocus photographic optical system whose F-number is about 1.4, the amount of various aberrations that may occur in a rear lens unit having a positive refractive power, which is disposed closer to the back side of the optical system than a front lens unit having a negative refractive power and an aperture stop, may be greater than that occurring in the front lens unit having a negative refractive power.
Therefore, it is difficult to effectively and balancedly correct various aberrations occurring in a retrofocus photographic optical system. In addition, in a retrofocus photographic optical system whose F-number is about 1.4, the effective diameter of the optical system may be likely to become large. Particularly, if the distance between the aperture stop and the rear lens unit becomes long, the effective diameter of the rear lens may increase. In this case, it becomes difficult to mount the lens barrel including the photographic optical system on the camera.
In order to solve the above-described matter, it may be useful if the positive lens constituting the rear lens unit, which is disposed closer to the back side of the photographic optical system than the aperture stop, is made of a glass material having a high refractive index.
If the above-described configuration is employed, it becomes easy to effectively correct various aberrations with a small number of lenses. In addition, in this case, the distance between the aperture stop and the rear lens unit can be reduced. Accordingly, the effective diameter of the rear lens can be reduced.
However, if the rear positive lens, which is disposed closer to the back side of the photographic optical system than the aperture stop, is made of a glass material having a high refractive index, then axial chromatic aberration and chromatic aberration of magnification may increase. This is because generally, the degree of dispersion of a glass material having a high refractive index is high (i.e., the Abbe number of a glass material like this is small).
More specifically, this is because as the angle of view of a photographic optical system becomes wider, the amount of axial chromatic aberration (longitudinal chromatic aberration) and chromatic aberration of magnification (lateral chromatic aberration) may become greater.
For the above-described reasons, in a retrofocus photographic optical system, it is difficult to form a high-quality image on the entire image plane while implementing a wide angle of view and a long back focal length.